FACTORS INFLUENCING CORN FERTILIZER N REQUIREMENTS IN THE NORTHERN UNITED-STATES CORN BELT

Nitrogen fertilizer recommendations must consider all potential sources of available N, as well as soil properties, fertilizer management, and climate effects to accurately estimate crop fertilizer N requirements. Systems approaches provide the possibility of evaluating N dynamics over a range of soil, crop, climate and management conditions. Our objectives were to use a systems model to evaluate the factors influencing corn fertilizer N requirements in soil-climate conditions typically encountered in the northern U.S. Corn Belt. Aboveground crop N requirements at near maximum yields ranged from 190 to 240 lb N/acre on an irrigated loamy sand soil and two rainfed silt loam soils. At equivalent yield levels, crop N requirements were an average of 25 lb N/acre lower on the irrigated loamy sand soil than on the two silt loam soils. Amounts of N in corn stover at maximum yields averaged 86 lb N/acre. Cumulative N uptake by corn grown on unfertilized control plots was used as a measure of soil N mineralization. A net mineralization rate of 3.5% of soil organic N during the growing season was estimated. This rate varied little among soil types. Apparent N contributions from soybeans or alfalfa to first-year corn, and rotation effects, were largely related to soil type and the frequency of legumes in the rotation. Apparent fertilizer N recoveries in corn grain decreased with increasing N application rates. At rates of N normally required to maximize grain yield, crop fertilizer N recovery (based on grain N recovery and aboveground N distribution) was greater on the irrigated loamy sand soil (50 to 67%) than on the silt loam soils (25 to 50%). Relatively large available N inputs from soil N mineralization and residual inorganic N on rainfed silt loam soils emphasizes the importance of accurate assessments of soil N contributions on these and similar soil types. On irrigated sandy soils, however, the high probability of leaching, combined with large fertilizer N demands, make N fertilizer placement and timing and effective irrigation management important considerations in promoting efficient N use and maintaining groundwater quality.